For transmission of digital signals, there is available a modulation technique called “OFDM” (orthogonal frequency division multiplex). The OFDM technique is such that data is digitally modulated for transmission by dividing a transmission frequency band into many orthogonal sub-carriers and assigning the data to the amplitude and phase of each of the sub-carriers by the phase shift keying (PSK) and quadrature amplitude modulation (QAM).
The OFDM technique is characterized in that since a transmission frequency band is divided into many sub-carriers, so the band per sub-carrier is narrower and the modulation rate is lower, while the transmission rate is not totally so different from that in the conventional modulation technique. The OFDM technique is also characterized in that since many sub-carriers are transmitted in parallel, so the symbol rate is lower and the time length of a multipath in relation to that of a symbol can be reduced so that the OFDM technique will not easily be affected by the multipath fading.
Also, the OFDM technique is characterized in that since data is assigned to a plurality of sub-carriers, so a transmission/reception circuit can be formed from an inverse fast Fourier transform (IFFT) calculation circuit in order to modulate the data, while it can be formed from a fast Fourier transform (FFT) calculation circuit in order to demodulate the modulated data.
Because of the above-mentioned characteristics, the OFDM technique is frequently applied to the digital terrestrial broadcasting which is critically affected by the multipath fading. To the digital terrestrial broadcasting adopting the OFDM technique, there is applied the Digital Video Broadcasting-Terrestrial (DVB-T) standard, Integrated Services Digital Broadcasting-Terrestrial (ISDB-T) standard or the like, for example.
As shown in FIG. 1, the transmission symbol used in the OFDM technique (will be referred to as “OFDM symbol” hereunder) is formed from an effective symbol as a signal duration for which IFFT is effected for transmission of data, and a guard interval as a copy of the waveform of an end portion of the effective symbol. The guard interval is provided in the leading portion of the OFDM symbol. Owing to such a guard interval, the OFDM technique allows a multipath-caused inter-symbol fading and improves the multipath resistance.
In the mode 3 of the ISDB-TSB standard (broadcasting standard for the digital terrestrial broadcasting, adopted in Japan), the effective symbol includes 512 sub-carriers spaced 125/126 kHz (≈0.992 kHz) from one to a next one. Also in the mode 3 of the ISDB-TSB standard, transmission data is modulated to 433 of the 512 sub-carriers in the effective symbol. Further in the mode 3 of the ISDB-TSB standard, the length of time of the guard interval is ¼, ⅛, 1/16 or 1/32 of that of the effective symbol.
For the OFDM technique, there is defined a unit of transmission called “OFDM transmission frame” normally composed of a plurality of OFDM symbols. Also, for the OFDM technique, there are defined positions in which a pilot signal of a predetermined phase and amplitude (called “continual pilot (CP)” or “scattered pilot (SP)”), transmission and multiplex configuration control (TMCC) information and transmission parameter signaling (TPS) information are to be inserted. With the OFDM technique, it is possible to control a variety of control, demodulation and decoding at the receiver side by transmitting such a pilot signal (CP or SP), transmission control information (TMCC or TPS) and the like.
According to the ISDB-TSB standard, one OFDM transmission frame is formed from 2040 OFDM symbols.
A conventional OFDM receiver will be illustrated and described.
FIG. 2 schematically illustrates the conventional OFDM receiver in the form of a block diagram.
As shown in FIG. 2, the conventional OFDM receiver, generally indicated with a reference 100, includes an antenna 101, tuner 102, band-pass filter (BPF) 103, A-D conversion circuit 104, DC canceling circuit 105, digital orthogonal demodulation circuit 106, FFT calculation circuit 107, frame extraction circuit 108, synchronization circuit 109, carrier demodulation circuit 110, frequency deinterleaving circuit 111, time deinterleaving circuit 112, demapping circuit 113, bit deinterleaving circuit 114, depuncture circuit 115, Viterbi circuit 116, byte deinterleaving circuit 117, spread-signal canceling circuit 118, transport stream generation circuit 119, RS decoding circuit 120, transmission-control information decoding circuit 121, and a channel selection circuit 122.
A transmission wave sent from a broadcast station is received by the antenna 101 of the OFDM receiver 100 and supplied as an RF signal to the tuner 102.
The RF signal received by the antenna 101 is converted in frequency by the tuner 102 composed of a multiplier 102a and local oscillator 102b into an IF signal, and the IF signal is supplied to the BPF 103. The oscillation frequency of a reception carrier signal generated by the local oscillator 102b is changed correspondingly to a channel select frequency supplied from the channel selection circuit 122.
The IF signal from the tuner 102 is filtered by the BPF 103, and then digitized by the A-D conversion circuit 104. The digital IF signal thus produced has the DC component thereof canceled by the DC canceling circuit 105, and is supplied to the digital orthogonal demodulation circuit 106.
The digital orthogonal demodulation circuit 106 makes orthogonal demodulation of the digital IF signal with the use of a carrier signal of a predetermined frequency (carrier frequency) to provide a baseband OFDM signal. The orthogonal demodulation of the baseband OFDM signal provides a complex signal composed of a real-axis component (I-channel signal) and an imaginary-axis signal (Q-channel signal). The baseband OFDM signal from the digital orthogonal demodulation circuit 106 is supplied to the FFT calculation circuit 107 and synchronization circuit 109.
The FFT calculation circuit 107 makes FFT calculation of the baseband OFDM signal to extract a signal having been orthogonal-modulated to each sub-carrier, and provides it as an output.
The FFT calculation circuit 107 extracts a signal having an effective symbol length from one OFDM symbol, and makes FFT calculation of the extracted signal. More specifically, the FFT calculation circuit 107 removes a signal having a guard interval length from one OFDM symbol, and makes FFT calculation of the residual of the OFDM symbol. Signals for FFT calculation may be extracted from any arbitrary positions in one OFDM symbol if the signal extraction points are consecutive. Namely, the signal extraction will start at any position in a range from the leading boundary of the OFDM symbol (indicated with a reference A in FIG. 1) to the end of the guard interval (indicated with a reference B in FIG. 1) as shown in FIG. 1.
A signal extracted by the FFT calculation circuit 107 and having been modulated to each sub-carrier is a complex signal composed of a real-axis component (I-channel signal) and an imaginary-axis component (Q-channel signal). The signal extracted by the FFT calculation circuit 107 is supplied to the frame extraction circuit 108, synchronization circuit 109 and carrier demodulation circuit 110.
Based on the signal demodulated by the FFT calculation circuit 107, the frame extraction circuit 108 extracts boundaries of an OFDM transmission frame, while demodulating pilot signals such as CP, SP, etc. included in the OFDM transmission frame and transmission-control information such as TMCC, TPS, etc., and supplies the demodulated pilot signals and transmission-control information to each of the synchronization circuit 109 and transmission-control information demodulation circuit 121.
Using the base-band OFDM signal, signals having been modulated to the sub-carriers after demodulated by the FFT calculation circuit 107, pilot signals such as CP, SP, etc. detected by the frame extraction circuit 108 and channel select signal supplied from the channel selection circuit 122, the synchronization circuit 109 calculates boundaries of the OFDM symbol, and sets an FFT-calculation start timing for the FFT calculation circuit 107.
The carrier demodulation circuit 110 is supplied with signals demodulated from the sub-carrier outputs from the FFT calculation circuit 107, and makes carrier demodulation of the supplied signal. For demodulation of an ISDB-TSB-based OFDM signal, for example, the carrier demodulation circuit 110 will make differential demodulation of the signal by the DQPSK technique or synchronous demodulation by the QPSK, 16QAM or 64QAM technique.
The carrier-demodulated signal undergoes frequency-directional deinterleaving by the frequency deinterleaving circuit 111, then time-directional deinterleaving by the time deinterleaving circuit 112, and is supplied o the demapping circuit 113.
The demapping circuit 113 demaps the carrier-demodulated signal (complex signal) to restore the transmission data series. For demodulation of an ISDB-TSB-based OFDM signal, for example, the demapping circuit 113 will make a demapping corresponding to the QPSK, 16QAM or 64QAM technique.
Being passed through the bit deinterleaving circuit 114, depuncture circuit 115, Viterbi circuit 116, byte deinterleaving circuit 117 and spread-signal canceling circuit 118, the transmission data series from the demapping circuit 113 undergoes deinterleaving corresponding to a bit deinterleaving for distribution of a multi-valued symbol error, depuncturing corresponding to puncturing done for reduction of transmission bits, Viterbi decoding for decoding a convolution-encoded bit string, deinterleaving in bytes, and energy despreading corresponding to the energy spreading, and the transmission data series thus processed is supplied to the transport stream generation circuit 119.
The transport stream generation circuit 119 inserts data defined by each broadcasting technique, such as null packet, in a predetermined position in a data stream. Also, the transport stream generation circuit 119 “smoothes” bit spaces in an intermittently supplied data stream to provide a temporally continuous stream. The transmission data series thus smoothed is supplied to the RS decoding circuit 120.
The RS decoding circuit 120 makes Reed-Solomon decoding of the supplied transmission data series to provide the transmission data series thus decoded as a transport stream defined in the MPEG-2 Systems.
The transmission-control information decoding circuit 121 decodes transmission-control information having been modulated in a predetermined position in the OFDM transmission frame, such as TMCC or TPS. The decoded transmission-control information is supplied to the carrier demodulation circuit 110, time deinterleaving circuit 112, demapping circuit 113, bit deinterleaving circuit 114 and transport stream generation circuit 119, and used to control the demodulation, reproduction, etc. effected in these circuits.
Note here that such an OFDM receiver has to synchronize OFDM symbols, digital orthogonal-demodulated carrier frequencies, and sampling clocks for the A-D conversion circuit, for example. For such kinds of synchronization, it is first necessary to detect a clock-frequency error.
The “clock-frequency error” is a difference in frequency between a transmission clock for a received OFDM symbol and a sampling clock for quantization of the received OFDM signal. That is, the clock-frequency error is a difference in frequency between a reference clock at the transmitter side and a reference clock at the receiver side.
A clock-frequency error can be detected by detecting an OFDM symbol boundary position through calculation of an autocorrelation of the OFDM signals and converting the boundary interval into a clock-frequency error, or by converting a position in which a pilot signal is inserted into an OFDM symbol period. In any case, however, it is difficult to detect a clock-frequency error accurately.